Spend a few minutes studying Figure 6.3.2–1, which shows major climate zones around the world . Although there is a lot of variation from place to place, you’ll also see some general patterns. For example, notice that wet, tropical regions are clustered near the equator, while dry, desert regions are found to the north and south of those wet regions.

As always in science, we take the existence of patterns like these as a hint that there must be underlying reasons behind them. In this section, we’ll explore these underlying reasons. We’ll organize our discussion by the three major climate factors that we discussed at the end of the last section: temperature, humidity and precipitation, and seasonal variation.

Temperature Zones

The fact that temperatures vary so strongly with latitude means it is useful to define different latitude zones around the world. Figure 6.3.2–3 shows the most common way of doing this:

• The tropics are the warm-weather latitudes that surround the equator, officially defined to represent latitudes between the Tropic of Capricorn (latitude 23½°S) and the Tropic of Cancer (latitude 23½°N). The tropics represent the latitudes at which Sun can sometimes pass directly overhead at noon . (Click here to review how the Tropics got their names.)
• The subtropics are regions that are within a few degrees of latitude of the tropics.
• Mid-latitudes (also called temperate zones) extend roughly from about latitude 30° to 60° in both hemispheres.
  Note that the subtropics and mid-latitudes are more vaguely defined than the tropics. For example, latitudes near 30° might be considered both the “upper” subtropics and the “lower” mid-latitudes.
• Polar zones are the Arctic and Antarctic regions. We usually think of them as anything north of about 60°N or south of about 60°S, but they are sometimes defined more precisely as the regions north of the Arctic Circle (latitude 66½°N) and south of the Antarctic Circle (latitude 66½°S).

Keep in mind that these latitude-based temperature zones tell us only general trends in temperature. There are many localized variations as a result of factors like how close a region is to the ocean, global wind patterns, and altitude. For example, Africa’s Mount Kilimanjaro is located very close to the equator (latitude 3°S), which puts it near the center of the tropics. However, it is so tall (5,895 meters, or 19,341 feet) that its peak is capped by snow and glaciers .

Precipitation Zones

Let’s turn now to precipitation, which also varies greatly around the world. Some regions are so wet that we call them rain forests, while others are dry enough that we call them deserts. Figure 6.3.2–5 shows how average (or “mean”) annual precipitation varies around the world. Study the figure, then try the quiz that follows to make sure you’ve noticed key patterns.

As you’ve hopefully learned from the quiz, the most obvious pattern in global precipitation is this:

• Heavy rainfall is concentrated fairly close to the equator, within the tropics . That’s why we refer to the rain forests of these regions (such as the large rain forests in South America, Africa, and Southeast Asia) as tropical rain forests .
• On either side of this tropical band, concentrated in the subtropics , we find the major desert regions of Earth.

Why do the wettest and driest regions of Earth come clustered in these latitude bands? You already know enough figure out the answer. Use the following questions to make sure you can put the full picture together.

Discuss the following questions with a classmate. Then click to open the answers to see if they agree with what you came up with.

3. Putting your answers from the first two questions together, explain why we should expect to find wet, rainy weather in regions near the equator.

4. Further building on your prior answers, now explain why we should expect to find dry, desert regions in the subtropics.

Figure 6.3.2–6 summarizes the picture you’ve put together from the questions above. Notice that the circulation cells create several bands of latitude that tend to have either low or high pressure. The low pressure bands, like the one near the equator, explain the locations of rain forests. The high pressure bands, like the ones in the subtropics, explain the locations of deserts.

Figure 6.3.2–6 – This figure shows how the three-cell circulation model leads to bands of high and low pressure that encircle the Earth at particular latitudes. The equatorial low pressure, called the intertropical convergence zone (ITCZ ), explains why rain forests are common in the tropics. The subtropical highs explain the presence of deserts at these latitudes.Credit: Adapted from The Cosmic Perspective.

The high and low pressure bands not only have dramatic effects on regional climates, but you can actually see them from space. The most clearly visible is the low-pressure band created where the equatorial cells meet, which is called the intertropical convergence zone , commonly abbreviated as ITCZ. Because the ITCZ is band of low pressure, we expect it to be lined with clouds. Figure 6.3.2–7 shows that this is indeed the case.

Figure 6.3.2–7 – The horizontal band of clouds in this photo traces out the location of the intertropical convergence zone (ITCZ) ,
a low-pressure band that encircles Earth near the equator. This band is created at the boundary of the north and south equatorial
circulation cells (as shown in Figure 6.3.2–6). It is marked by clouds because low pressure tends to mean cloudy skies.
Note: As discussed below, the precise location of the ITCZ shifts with the seasons. Here, it is slightly to the north of the equator,
which tells us that this photo was taken during northern hemisphere summer. Credit: NOAA.

Seasonal Shifts

You know that seasons are caused by the tilt of Earth’s axis, and that the angle of sunlight at different times of year is determined by latitude. Why then, would some places have stronger seasonal shifts in weather than other places at the same or similar latitudes? The full explanation is complex, but let’s look at two major global effects that are relatively easy to understand.

Seasonal Shifts in Circulation Cells

Discuss the following questions with a classmate. Then click to open the answers to see if they agree with what you came up with.

2. Does the greatest solar heating always occur right at the equator, or does it shift with the seasons? Explain.

3. Combining your answers to the first two questions, how would you expect the equatorial circulation cells to shift with the seasons?

Video 6.3.2–8 shows how the seasonal shifts in solar heating correspond to seasonal shifts in temperature. As you found in answering the questions above, the circulation cells must also move alternately northward and southward with these seasonal shifts.

The northward and southward seasonal shifts in the locations of the circulation cells mean that their boundary, which creates the low-pressure intertropical convergence zone (ITCZ) , also shifts in the same way. Figure 6.3.2–9 shows typical January and July locations for the ITCZ. Notice that the shift is not exactly the same everywhere around the world. The reason is that although solar heating depends only on latitude, the circulation cells and the ITCZ are also influenced by geography, such as the locations of continents and oceans and the distribution of mountain ranges.

Figure 6.3.2–9 – This map shows how the location of the low-pressure intertropical convergence zone (ITCZ) shifts at
different times of year. Credit: Mats Halldin.

The seasonal shifts in the circulation zones and the ITCZ are one part of explaining whether a particular region has strong seasonal changes in weather. Notice, for example, that the January and July positions of the ITCZ are fairly close together in northern South America, which explains why this region has a fairly steady climate all year round. In contrast, the ITCZ shifts a lot with the seasons near southeast Asia, leading to much greater seasonal differences in climate. Similar effects occur with the shifts of other circulation cells (the mid-latitude and polar cells), explaining some of the seasonal differences we see at higher latitudes.

The Monsoon
The seasonal shifts in the circulation cells have many consequences for weather and climate. One of the most important is the pattern that we call the monsoon . The word “monsoon” is sometimes used in other ways, but we’ll focus on its technical definition as a seasonally reversing wind. In brief, the monsoon pattern arises from the fact that land can warm or cool faster than the oceans (remember that the oceans have a moderating influence on climate) as follows:

• In summer months, the land warms more than the oceans, creating lower pressure over the land. Therefore, the wind blows from the oceans to the land, bringing moisture from the oceans that can create heavy rain.
• In winter months, the oceans remain warmer than the land, creating higher pressure over the land. Therefore, the wind blows from the land toward the oceans, leading to a dry season over the land.

Monsoons occur in many places around the world, but one of the strongest monsoon patterns occurs in southeast Asia, where it affects the lives of billions of people. Figure 6.3.2–10 shows the general monsoon pattern in this region for winter (left) and summer (right), and Video 6.3.2–11 uses satellite data to explain the pattern in a little more detail.

Summary and Additional Climate Factors

We can briefly summarize the climate factors that we’ve discussed so far as follows:

If you now look again at Figure 6.3.2–1, repeated below, you’ll see how these patterns explain many of the climate zones. However, there are still plenty of exceptions to the general trends. These arise from other factors, including the way oceans can moderate climate, the way mountains can create a rain shadow effect, and the fact that higher altitudes tend to be colder. Moreover, all of these factors interact with each other, making the complete climate system extremely complex. Nevertheless, by using basic principles like those we have discussed (and many more), scientists can now create computer models that can successfully explain most aspects of Earth’s climate. We’ll discuss these models in Section 7.2.2, where we will see how scientists are using them to help us understand and predict the future effects we can expect from global warming.

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